As is well-known in the art, an internal combustion engine draws in ambient air, mixes the air with fuel, and introduces the mixture of air and fuel into a combustion chamber, where the mixture of air and fuel is ignited and burned. The resulting exhaust gases, which may be treated to remove pollutants, are then expelled into the atmosphere. Ignition of the air/fuel mixture in the cylinder is typically achieved by an ignition device, typically, a spark plug or the like, or by the adiabatic compression of the air/fuel mixture, which heats the mixture to a temperature above its ignition point.
In gasoline powered internal combustion engines commonly in use today, ambient air is conveyed via an air intake duct or port to a carburetor or a fuel injection system, which is used to mix the air with the fuel to create the air/fuel mixture. For engines with some types of fuel injection systems, as well as those equipped with carburetors, the air/fuel mixture is then conveyed via an intake manifold to the combustion chamber or cylinder of the engine. In gasoline engines equipped with port injection type fuel injection systems, the air is directed through the intake manifold to the intake port of the combustion chamber before the fuel is mixed with the air. In diesel-type engines and some gasoline engines using fuel-injection systems, the air and fuel are conveyed separately to the combustion chamber or cylinder of the engine where they are mixed.
After the combustion of the air/fuel mixture, the resulting exhaust gases are expelled from the combustion chamber to an exhaust manifold. In almost all modern gasoline powered automobiles, the exhaust gases are then conveyed by an exhaust pipe to a catalytic converter where pollutants are substantially removed from the exhaust gas. However, during the operation of an internal combustion engine, even one equipped with pollution control devices, such as a catalytic convertor, some pollutants, as described below, remain in the exhaust stream, and are expelled into the atmosphere.
In addition to complete combustion products, such as carbon dioxide (CO.sub.2) and water (H.sub.2 O), internal combustion engines also produce exhaust gases containing a number of pollutants, e.g., carbon monoxide (CO), a direct poison to human life, and hydrocarbons (HC), that result from incomplete combustion. Also, due to the very high temperatures produced by the burning of the hydrocarbon fuels followed by rapid cooling, thermal fixation of nitrogen in the air results in the detrimental formation of nitrogen oxides (NO.sub.x), an additional pollutant.
The amount of CO, HC, NO.sub.x and other pollutants produced by an internal combustion engine varies with the design and operating conditions of the engine and the fuel and air used. In particular, the amount of CO, HC, and NO.sub.x pollutants is determined in part by the air-to-fuel ratio, such that conditions conducive to reducing carbon monoxide and hydrocarbons, i.e., a fuel mixture just lean of stoichiometric, which results in higher combustion temperatures, causes an increase in the formation of NO.sub.x, and conditions conducive to reducing the formation of NO.sub.x, i.e., fuel rich or fuel lean mixtures, which results in lower combustion temperatures, causes an increase in carbon monoxide and hydrocarbons in the exhaust gases of the engine.
Although the presence of pollutants in the exhaust gases of internal combustion engines has been recognized since 1901, the control of internal combustion engine emissions in the United States only became required by law with the passage of the 1970 Clean Air Act. Engine manufacturers have explored a wide variety of technologies to meet the requirements of this Act, including exhaust gas recirculation, electronically controlled fuel injection systems, which receive data from various sensors in the combustion stream, allowing the accurate control of the air/fuel ratio, and catalytic convertors. Catalysis has proven to be the most effective passive system.
The purpose of a catalytic convertor is to oxidize CO and HC to CO.sub.2 and H.sub.2 O, and, in a three way catalyst, to reduce NO/NO.sub.2 to N.sub.2. In modern three way catalytic converters (TWC) in which all three pollutants are reduced simultaneously, NO.sub.x reduction is most effective in the absence of oxygen, while the abatement of CO and HC requires oxygen. Therefore, the prevention of the production of these emissions requires the operation of the engine at or near the stoichiometric air-to-fuel ratio.
Today, nearly all automobile catalytic converters are noble metals, held in honeycomb monolithic structures, which have excellent strength and crack-resistance under thermal shock. The honeycomb construction and the geometries chosen provide a relatively low pressure drop and a large total surface area that enhances the mass transfer controlled reactions that remove pollutants from the exhaust. The honeycomb is set in a steel container, and protected from vibration by a resilient matting.
An adherent washcoat, generally made of stabilized gamma alumina into which the catalytic components are incorporated, is deposited on the walls of the honeycomb. TWC technology for simultaneously converting all three pollutants typically utilizes the precious or noble metals platinum (Pt) and rhodium (Rh), where the Rh is most responsible for the reduction of NO.sub.x, while also contributing to CO oxidation, which is primarily performed by Pt. Recently palladium, Pd, which is less expensive, has been substituted for or used in combination with Pt and Rh. The active catalyst generally comprises about 0.1 to 0.15% of these metals.
Because the exhaust gases of the combustion engine oscillate from slightly rich to slightly lean, an oxygen storage medium is added to the washcoat to adsorb oxygen onto the surface of the washcoat during any lean portion of the cycle, and release the oxygen for reaction with excess CO and HC during any rich portion of the cycle. Cerium Oxide (CeO.sub.2) is most frequently used for this purpose due to its desirable reduction-oxidation response.
The passage of the 1990 Amendment to the Clean Air Act requires significant further reductions in the amount of pollutants being released into the atmosphere by internal combustion engines. In order to comply with these requirements, restrictions on the use of automobiles and trucks have been proposed, such as, employer-compelled car pooling, HOV lanes, increased use of mass transit as well as rail lines and similar actions limiting automobile and truck usage at considerable cost and inconvenience.
An alternative to diminished automobile and truck usage is decreasing emissions by increasing the efficiency of the internal combustion engine. This approach will have limited impact since studies show that most of automobile-originated pollution is contributed by only a small fraction of the vehicles on the road, these vehicles typically being older models having relatively inefficient engines and aging catalytic converters which inherently produce a lot of pollution. Any technological improvements to the total combustion process will not be implemented on these older vehicles if they require extensive or expensive modification to the engine or vehicle.
In addition, while considerable gains have been made in recent years to reduce the amount of pollutants in the exhaust gases of the internal combustion engine of vehicles such as automobiles and trucks, further reductions in the amount of pollutants in the exhaust gases of the internal combustion will be expensive, and presents a considerable technological challenge, since exhaust emissions of automobiles and trucks currently being manufactured do not meet proposed Environmental Protection Agency standards.
In lieu of decreasing exhaust emissions by increasing the efficiency of the internal combustion engine or decreasing the use of automobiles, a further alternative would be to increase the efficiency of the catalytic converter or catalysis. The conversion efficiency of a catalytic converter is measured by the ratio of the rate of mass removal of the particular constituent of interest to the mass flow rate of that constituent into the catalytic converter. The conversion efficiency of a catalytic converter is a function of many parameters including aging, temperature, stoichiometry, the presence of any catalyst poisons, such as lead, sulfur, carbon and phosphorous, the type of catalyst, and the amount of time the exhaust gases reside in the catalytic converter.
Prior art attempts to increase the efficiency of catalytic converters have not been sufficiently successful. Modern TWC catalytic converters help, but they are expensive, may have difficulty in meeting the future emission requirements, and have limitations in their performance lifetime. Catalytic converters also suffer from the disadvantage that their conversion efficiency is low until the system reaches operating temperature.
Therefore, a need exists for a simple, inexpensive means of reducing the amount of pollution released by internal combustion engines and catalyst combination that can be installed on engines that are presently in use, as well as newly manufactured engines. The present invention provides such a means.